Spitzer Space Telescope, originally known as the Space Infrared Telescope Facility (SIRTF), was an orbiting observatory that studied the early universe, distant galaxies and newly forming stars and planets. It was also the fourth and final mission of NASA's Great Observatories, a series that has included the Hubble Space Telescope, the Chandra X-Ray Observatory and the Compton Gamma Ray Observatory.

The observatory consisted of an 85-centimeter-diameter (33.5-inch) telescope and three cryogenically cooled science instruments. The science instruments performed imaging and spectroscopy -- breaking light into different colors, like a prism, so the chemical composition of an object can be determined. Observations were conducted in the infrared to detect objects and processes in the universe that are either too cool or too dust-enshrouded to be seen otherwise. The observatory launched into an orbit that trailed Earth as it goes around the Sun. This unique orbit placed the observatory in deep space, far from the heat generated by Earth itself.

Spitzer also was unique in that it was smaller and lighter than past missions involving cryogenically cooled telescopes. In past missions, a vacuum shell surrounded the telescope like a thermos bottle, actively chilling the observatory and science instruments. On Spitzer, the vacuum shell surrounds only the instrument chamber and the liquid helium tank. Engineers refer to this configuration as a warm launch architecture. It means that much less coolant is needed, allowing for the use of a relatively smaller launch vehicle. The configuration allowed Spitzer to collect science data for more than five years -- twice the length of the longest previous infrared mission.

Spitzer was only one-third as long and one-eleventh the weight of its bigger sibling, the Hubble Space Telescope. The four-meter-tall (14.6-foot), 865-kilogram (1,907-pound) observatory enclosed a cryogenically cooled telescope and three science instruments - a multiband imaging photometer, an infrared spectrometer and an infrared-detector array camera - as well as all the required power, computer, communications and navigation equipment.

The telescope was cooled using extremely cold liquid helium. Over the course of the mission, this ultra-cold helium slowly leaked out into space. After five and a half years, all of the helium was lost and the spacecraft was reconfigured for an extended mission observing warmer targets.

The heart of Spitzer was an 85-centimeter-diameter (about 33.5-inch) telescope and three cryogenically cooled science instruments that performed imaging and spectroscopy at wavelengths from about 3 to 180 microns.

Like the Hubble Space Telescope, Spitzer was a Cassegrain design, named for the French sculptor Guillaume Cassegrain, who invented it in 1672. Light from distant objects in space enters the telescope and is reflected by a primary mirror at the telescope's rear. The light is then gathered onto a smaller secondary mirror suspended in the middle of the telescope near the front end. The light in turn reflects back toward the rear of the telescope, where it passes through a hole in the middle of the primary mirror. At the rear, behind the primary mirror, is the sensor that records the image. Spitzer's telescope weighed less than 50 kilograms (about 110 pounds).